BEIJING, Nov. 25 -- The chief organizer of a six-day AIDS conference scheduled to begin Sunday has been detained by men who identified themselves as Beijing security forces, according to a colleague at his organization.

Wan Yanhai, 43, is a former Health Ministry official and a leading AIDS activist in China, which has one of the highest HIV growth rates in the world. Although the government has become more active in promoting AIDS awareness in recent years, officials still harass activists and suppress information that exposes corrupt local officials.

Wan was last heard from on Friday evening, hours after telephoning his colleague to say the conference would be canceled, the colleague said, speaking on condition of anonymity for fear of retribution.

The conference was to have provided a platform for legal professionals and nongovernmental organizations to discuss AIDS and blood-transfusion issues, according to the coordinator for the Zhiaixing Information Counseling Center, formerly known as the Aizhixing Health Education Institute. Sponsors of Aizhixing -- which means love, knowledge, action -- include the Open Society Institute, the National Endowment for Democracy, the British Embassy, Harvard University and AIDS organizations in Canada, France and Australia.

Wan and his colleague were called to the property management office of their office compound in the Beijing neighborhood of Haidian about 10:30 a.m. Friday.

"They claimed that they were from Beijing Public Security Bureau, but they didn't offer any identity certificate. They asked if Wan was the person responsible for Aizhixing, and after they confirmed that, they firmly asked me to leave, saying they wanted to talk to Wan in private," the colleague said.

The property management staff said the men were from the criminal investigation section of the Haidian District Public Security Bureau and that they did show their credentials, said another colleague, who also spoke on condition of anonymity.

The first colleague said he last heard from Wan at around 1 p.m. Friday, when Wan telephoned to say that the AIDS conference was canceled. "He called another colleague later, and we could not get in touch with him after that," the first colleague said.

Wan has worked to promote AIDS awareness throughout China, including in Henan province, where officials ignored and contributed to the spread of the disease by encouraging peasants to sell their blood for cash.

In 2002, Wan was detained after he distributed a government report that showed officials knew about a brewing AIDS epidemic in Henan years before they acknowledged its existence. Wan confessed to revealing state secrets and was held for 26 days.